1. Field of the Invention
The present invention relates to a radar system, mounted on a vehicle for example, and used for calculating a distance etc. to an object by using an electromagnetic wave, and more specifically, to a method of judging for the presence of contamination on a radome.
2. Description of the Related Art
In general, a radio-wave type radar system has such a merit that the radar system can be mounted on an inside of an exterior of a vehicle without impairing a design of the vehicle because of characteristics of its medium.
Therefore, in many cases, the radar system is mounted on the inside of the exterior of the vehicle, and a radome for protecting an antenna of the radar system is mounted on the exterior of the vehicle.
When contamination such as a film of water containing snow and dirt for example is present on the radome at this time, the electromagnetic wave transmitted from the antenna is reflected on the surface of the radome without transmitting through the radome and the antenna receives the reflected wave from the radome.
Therefore, the radar system receives the reflected wave from a very close distance, causing a problem in that accuracy of calculation for calculating the distance etc. to the object drops.
In order to solve the above-mentioned problem and to improve the accuracy of the calculation, a conventional FM radar system transmits a FM signal sweeping frequencies, receives a reflected signal from an object, and detects position of the object from a frequency of a be at signal obtained by mixing a part of the reflected and transmission signals. The FM radar system has means for detecting contamination adhering to a radome of the antenna based on a level of a low-frequency component of the be at signal and processes the be at signal by two types of filters of a band-pass filter and a low-pass filter. The system detects the position of the object based on an output of the band-pass filter and detects the contamination on the radome based on an output of the low-pass filter (see, for example, JP 1998-282229 A).
The conventional system described above detects that the contamination is present on the radome by comparing frequency spectrum data of the low frequency component obtained by converting the low frequency component of the be at signal from analog to digital and implementing fast Fourier transform with frequency spectrum data of the low frequency component when the radome is not contaminated.
Herein, as reflected wave components of a very close distance other than the reflected wave from the surface of the radome caused by the contamination adhering to the radome, there exist reflected wave components of the close distance caused by a leak between transmission and reception within a high frequency circuit or caused by a leak between the transmission and reception antennas.
Levels of those reflected wave components changes depending on individual differences of the radar system and on temperature because it is determined by a phase relationship with the transmission wave component to be mixed.
The conventional FM radar system has a problem in that its accuracy for detecting the contamination adhering to the radome is reduced because the conventional FM radar system gives no consideration to the reflected wave component in the close distance.
In view of the above, in order to solve the above-mentioned problem and to improve the accuracy of detection, there can be considered a method of providing a plurality of maps of criterion values by taking temperature characteristics into consideration and of detecting the contamination adhering to the radome based on these maps.
However, the method has a problem in that adjustment costs increase because each individual radar system must be provided with the maps of the criterion value.
There can be also considered a method of always learning the reflected wave components in the close distance, storing learnt values and of detecting contamination adhering to the radome based on variations from the learnt values.
However, the method has a problem in that the radar system cannot detect the variations and thus cannot detect the contamination because when the contamination adheres to the radome while the radar system is in a halt, for example, the aforementioned learning starts in the state where the contamination is present on the radome when an engine starts.